1. Field of the Invention
The present invention relates to an image forming apparatus and more particularly an image forming apparatus configured to obviate contamination ascribable to toner having a small grain size.
2. Description of the Background Art
A copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus is conventional and includes an image carrier. A developer containing toner is used to develop a latent image formed on the image carrier for thereby producing a corresponding toner image. The toner image is transferred to a sheet or recording medium and then fixed thereon. A dry-process developing device, which uses a dry developer, develops the latent image by using any one of a cascade method, a magnet brush method, a powder cloud method and other conventional methods. The developer for the dry-process developing device is either one of toner only, i.e., a one-ingredient type developer and a toner and carrier mixture, i.e., a two-ingredient type developer.
It is a common practice with the developing device to stabilize image density by maintaining the toner content of the developer constant, i.e., by replenishing fresh toner at adequate timing. On the other hand, to promote the efficient use of limited resources, Japanese Patent Laid-Open Publication No. 60-41079, for example, proposes to collect toner left on an image carrier after image transfer and return it to the developing device. Japanese Patent Laid-Open Publication Nos. 6-175488 and 11-352761, for example, each disclose a particular toner recycling structure including a pipe extending from a cleaning device assigned to the image carrier to the developing device. A screw conveyor is disposed in the pipe for conveying the collected toner from the image carrier to the developing device.
On the other hand, lowering the melting point of toner is successful to promote rapid melting and infiltration of the toner during fixation and therefore to reduce fixing time. When toner with a low melting point is used, additives each may be coated on toner grains for a particular purpose, i.e., for preventing the fluidity of toner grains from being lowered or for increasing or reducing the amount of charge to deposit on the toner grains, as taught Japanese Patent Laid-Open Publication No. 6-175488 by way of example. The toner grains may additionally contain carnauba wax as a parting agent, as also taught in this document.
Today, the grain size of toner is sometimes reduced in order to enhance dot reproducibility and sharpness. For example, when the recycling system stated earlier is used and when operation speed is increased to reduce image forming time, use is made of toner with a small grain size in order to faithfully reproduce a dot image. More specifically, a smaller grain size makes dots less irregular in shape and thereby renders a halftone image more smooth. Further, for a given mean grain size, the greater the content of fine grains, the higher the dot reproducibility. However, some problems to be described later specifically are left unsolved when reproducibility is enhanced by use of toner with a small grain size and a great fine grain content.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2000-39740, 2000-172005, 2000-181128 and 2000-194161 and Japanese Patent No. 3,174,984.
It is an object of the present invention to provide an image forming apparatus capable of obviating, when use is made of toner with a small grain size, background contamination, toner scattering and sleeve contamination without regard to operation speed or environmental conditions.
In accordance with the present invention, in an image forming apparatus of the type developing a latent image formed on an image carrier, which moves at a linear velocity of 400 mm/sec or above, with a developer containing toner grains having a volume mean grain size of 5 xcexcm to 10 xcexcm and 60 number percent to 80 number percent of which has a grain size of 5 xcexcm or below, the developer contains carrier grains having a weight mean grain size of 65 xcexcm or below.